The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Manufacturers of internal combustion engines are continuously developing new engine control strategies to satisfy customer demands and meet various regulations. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such engines include both compression ignition (diesel) and lean-burn spark-ignition engines. When an engine operates in a region lean of air/fuel stoichiometry, this typically results in increased combustion temperatures, which leads to increased oxides of nitrogen (NOx) emissions.
One proposed type of exhaust aftertreatment system and control strategy for managing and reducing NOx emissions involves the injection of a dosing agent such as diesel exhaust fluid (DEF) into an exhaust gas feedstream entering a selective catalytic reduction (SCR) device. DEF comprises a solution of urea and deionized water, which decomposes into ammonia (NH3) when heated in the exhaust gas feedstream. A typical SCR device has a capacity to store the ammonia resulting from the decomposed urea on its catalyst surface. The NOx in the exhaust gas passing through the SCR is reduced by the stored ammonia on the catalyst surface into nitrogen gas (N2), water (H2O), and small amounts of carbon dioxide (CO2), which are passed out of the SCR device.
The SCR device is able to continue NOx reduction using the stored ammonia when the dosing injection system is not supplying urea. The maximum ammonia storage capacity of the SCR device is inversely related to its operating temperature, which can be determined empirically.
This SCR process works reasonably well, provided the SCR catalyst is maintained at the right temperature (approximately 570° to 750° F.), and the correct amount of urea is injected and stored as ammonia in the SCR device for reducing NOx in the exhaust feedstream. If too little ammonia is stored in the SCR device as compared to the amount of NOx in the exhaust feedstream, conversion efficiency will drop, and undesirable NOx emissions exiting the exhaust aftertreatment system will increase. Conversely, if the maximum ammonia storage capacity of the SCR device is exceeded, an undesirable phenomenon known as ammonia slip will occur, where unprocessed NH3 exits the SCR device.
Additionally, if the operating temperature of the SCR device increases rapidly at a time when ammonia storage is near its maximum, ammonia slip can also occur due to the inverse relationship between the temperature of the SCR device and its maximum ammonia storage capacity. This can occur, for example, when exhaust gas temperature increases rapidly due to heavy accelerator pedal tip-in by the vehicle operator.
Conventional methods for controlling ammonia storage in SCR devices rely on various sensors, which attempt to measure real-time values of various engine operating parameters and exhaust gas parameters. Such conventional control systems are reactionary as they have to react to changes in operating parameters after the fact, which results in less than optimal control.
Accordingly, there exists a need in the art for more effective control of ammonia storage in SCR devices for improving NOx emission conversion and reducing ammonia slip.